Dish antenna structures and hydraulic control of the orientation thereof

ABSTRACT

A dish antenna framework is constructed from tetrahedral strut assemblies. Each assembly is made up of six struts joined at their ends to four nodes. The nodes also constitute dish mounting points for the dish on the dish antenna framework. These points are located on the envelope of the dish. The framework supported on base frame is rotated about the vertical axis using a movable hydraulic ram arrangement wherein a ram cylinder is attached by an arm to the base frame to be rotated, a ram piston is removably attached to anchor members fixed to a platform. Rotation is effected by expanding and contracting the hydraulic ram. The ram is guided between anchor members by guiding means.

TECHNICAL FIELD

This invention concerns rigid structures. It also concerns dish antennasof the type used for radio telescopes, solar collectors, satellitecommunication, and the like. In particular, this invention concernsstructures for supporting the dishes of such, preferably using a novelhydraulic arrangement for the controlled rotation of the dish supportingstructure about a vertical axis.

BACKGROUND OF THE INVENTION

Dish-like antennas which are used as, for example, receivers of signalsfrom satellites, solar collectors and radio telescopes, utilise areflecting dish to focus electro-magnetic radiation upon a receiver. Thedish comprises a reflective or conductive surface which is mounted on arigid frame. The dish, with its supporting frame, is manipulated by oneof a number of conventional techniques, discussed below, to pointcontinuously to the object from which the antenna receiveselectro-magnetic radiation.

In a typical dish antenna, the frame which supports the reflector dishis usually a complex structure. It may be an inverted geodesic dome, ora series of concentric hoops supported by a plurality of identicalsub-frames extending radially from below the centre of the dish. Suchbasic dish support frames are relatively weak structures and quitecomplex bracing arrangements are required to strengthen them. Even withsuch bracing, if the dish has a continuous surface (which is usually thecase when the dish is used to receive and focus solar radiation), it candistort to a significant extent when the antenna is subjected tomoderate wind loads.

A further disadvantage of the existing dish supporting frames is that,unless detailed and time-consuming design procedures are used to producethe support frame design, they are not constructed so that the points(called "mounting points") on the support frame to which the dishsurface is attached lie accurately on the envelope of the surface of thedish. Thus, when assembling the antenna structure, and in particularwhen mounting a large reflecting surface on the dish supporting frame,it is usually necessary to adjust the mounting of each portion of thereflecting surface to form the required surface shape of the dish. Thecost of producing the existing dish antennas is thus quite high, partlydue to the complex configuration of the supporting frame and partly dueto the amount of skilled labour required in the assembly of the antennastructure.

As mentioned above, the distortion of the dish surface under evenmoderate wind loads is particularly significant in the case of antennasused as solar collectors, which have continuous reflector surfaces andwhich need to be cost effective. Such antennas must be capable ofreceiving radiation from the sun even when the sun is at the horizon.

Since a change in the elevation of the line of sight of a dish iseffected by movement of the dish and its associated support frame abouta horizontal axis which is positioned below the centre of the dish whenthe dish is pointing directly upwards, the axis of rotation of the dishstructure must be at a distance above the ground that is at least halfthe vertical extent of the dish, measured when the dish has its line ofsight directed to the horizon. This horizontal axis of rotation isinvariably at the top of a tower. Thus, when the dish is moved so thatits line of sight is vertically above this axis, all of the dish surfaceis positioned well above the ground, where it is fully exposed to thewind. In other than light breezes, the wind loads will distort areflector dish surface unless the support frame is a complex structurewhich includes a number of bracing members and the dish is constructedfrom a strong, and therefore heavy, material (in which case there mustbe an increase in the handling capabilities of ancillary equipment). Ifthe dish is not made from a heavy, rigid material, it may be necessaryto curtail the operation of the antenna in strong winds to avoid thepossibility that the dish will be damaged. In either case, an economicpenalty is incurred.

A further problem associated with large dish antennas is associated withthe rotation of the antenna structure about a vertical axis, which is anintegral part of the "tracking" of the antenna to keep its line of sightdirected at a particular object (the sun, in the case of a solarcollector). Normally, this rotation of the antenna structure is effectedusing a motor which drives a pinion. The pinion engages with an arcuateor circular toothed track. The motor, which is electrically orhydraulically powered, drives the pinion through a reduction gearbox, sothat the antenna is rotated continuously, yet slowly, about a verticalaxis. Such electric or hydraulic motors, with their reduction gearboxes,are expensive components. Furthermore, should there be a power failure,provision must be made to "off steer" the antenna rapidly to avoidpotential damage to the receiver of electromagnetic radiation. The"off-steering" device requires a back-up power supply to enable it tofunction in an emergency. The back-up power supply is usually a bank ofbatteries, which require regular maintenance as well as adding to thecapital cost of the antenna installation.

DISCLOSURE OF THE PRESENT INVENTION

It is an object of the present invention to provide a support structurefor the dish of a dish antenna which is more rigid than the conventionaldish-supporting frame, yet which is also a relatively light-weightconstruction.

This objective is achieved by the provision of a dish supporting framewhich (a) is constructed as a number of strut assemblies, each strutassembly being a tetrahedral arrangement of six rigid struts, the endsof which are connected to four "nodes", or joining points, and (b) has aplurality of dish mounting points to which the dish, or the elementswhich form the dish, may be attached. The dish mounting points are onthe envelope of the dish surface (which will usually be a paraboloidalsurface or a surface having the shape of the cap of a sphere--generallytermed a "spherical surface").

The use of tetrahedral strut assemblies provides a strong and rigid, yetrelatively light-weight, supporting frame for the dish of the antenna.In addition, careful selection of the length of the struts in thevarious tetrahedral strut assemblies permits the mounting points for thedish, which are at respective nodes of the strut assemblies, to bepositioned accurately in any required location. By having the mountingpoints at the envelope of the dish surface, the dish--or the elementswhich make up the dish--can be mounted on the support frame without theneed for adjustment of the spacing between each mounting point and thedish or dish element. Thus the antenna design can be effected in alaboratory and provided the appropriate number and lengths of struts aresupplied to the antenna site, the dish supporting frame can be assembledaccurately, in situ, and the dish can be mounted on the frame and theantenna operated immediately.

The present inventors have appreciated that the tetrahedral strutassemblies, used to create a rigid dish support frame for a dishantenna, can also be used to create beams or towers having goodtorsional stability. Such beams (for example, for use as a crane arm)and towers are formed by a plurality of at least three rod members,interconnected by a plurality of strut assemblies, each strut assemblycomprising a tetrahedral assembly of six struts, joined at their ends tofour nodes (to some of which the rod members are rigidly connected).

Reverting to the dish antenna realisation of the present invention, itis normal practice to mount the dish support frame on a main frame ofthe antenna. The mounting of the support frame is effected at ahorizontal tilt axis, about which the support frame (and thus the dish)is rotatable. A preferred feature of the present invention is theprovision of an elevation tilt axis between the central portion and theoutermost ends of the strut assemblies of the dish support frame (thatis, at a location beneath the dish of the antenna, intermediate betweenthe centre of the dish and its periphery). With this feature present inthe antenna structure, the total height of the antenna when its line ofsight is vertical is less than the total height of a conventional dishantenna of the same dish size, arranged with its line of sight vertical.Thus the wind loading on an antenna having this preferred feature isreduced.

In a preferred form of the present invention, the support frame for thedish of a dish antenna is also mounted for rotation about a verticalaxis. The preferred mechanism for such rotation of the support frameinvolves an arm which extends from the vertical axis of rotation of thebase frame of the antenna (this arm is rigidly connected to orconstitutes a component of the base frame of the antenna, on which thedish supporting frame is mounted) to the cylinder of a double-actinghydraulic ram. The free end of the rod of the hydraulic ram is connectedto one of a plurality of anchor members, which are rigidly mounted onthe earth or on the platform on which the base frame is mounted. Theanchor members are approximately equi-spaced around a circle having itscentre at the vertical axis of rotation of the antenna. When thehydraulic ram is contracted, it pulls the end of the arm towards theanchor member to which the end of the ram rod is connected, and thus torotates the antenna about its vertical axis. At a predetermined point inthe contraction of the ram, the contraction ceases, the arm is locked inthe position it has reached, and the free end of the ram rod isdisconnected from the anchor member. Then the ram is expanded, and thefree end of the ram rod is guided to the next anchor member of thecircle of anchor members. Upon arrival at the next anchor member, theexpansion of the ram is ceased, the free end of the ram rod is connectedto the next anchor member, and the antenna arm is unlocked. The ram isthen contracted to re-start the movement of the arm, and thus of theantenna, about the vertical axis.

Reversing this sequence will cause the antenna to be slowly rotatedabout its vertical axis in the opposite direction.

Since the movement of the free end of ram rod can be effected rapidlywhile the arm is locked, there is no significant interruption of theslow rotational movement of a tracking antenna. In fact, for largeantennas used as solar collectors, for which this aspect of the presentinvention was developed, the dimensions of the components of the solarenergy collection arrangement are such that stepwise contraction of thehydraulic ram is normally used to rotate and reposition the antennastructure. This will be explained in more detail later in thisspecification.

Not only is such a rotational drive mechanism substantially less costlythan the conventional drive motor and its assoicated accurately laidtrack with which the pinion driven by the drive motor engages, but theemergency "back up" arrangement for off-steering the antenna can be thesame ram arrangement adapted to be driven by pressurised gas (forexample, nitrogen) from a cylinder of the gas.

It will be appreciated that this method for rotating a dish antenna canbe used to rotate other bodies.

Thus, according to the present invention there is provided a dishsupport frame for the dish of a dish antenna, said dish support framecomprising

(a) a plurality of strut assemblies, each strut assembly comprising sixrigid struts, connected at their ends to four nodes of the support frameto form a tetrahedral strut assembly; and

(b) a plurality of dish mounting points, each dish mounting point beingat a respective one of the nodes of a strut assembly, said dish mountingpoints being located on the envelope of the shape of the dish.

As noted above, preferably the dish support frame has a pivotalconnection to the base frame of the antenna to permit the elevation ofthe line of sight of the antenna to be varied, this pivotal connectionbeing at a tilt axis which is located away from the centre of the dishsupport frame and preferably between the centre of the dish supportframe and the periphery of the dish support frame. The tilt axis couldbe outside the edge of the dish support frame, though it is believedthat a tilt axis in such a location will rarely be required.

In a preferred form of the present invention, there is provided a dishsupport frame for the dish of a dish antenna, as defined above, thesupport frame being mounted for pivotal movement about a horizontal axiswhich is displaced laterally from the center of the support frame.

In a further preferred form of the present invention, the dish supportframe is mounted for rotation about a vertical axis, using a mechanismcomprising

(a) an arm attached to or forming part of said body, and extendinggenerally radially from said vertical axis;

(b) a hydraulic ram having a ram cylinder and a ram rod actuated byhydraulic fluid within said ram cylinder, said ram cylinder beingconnected to said arm;

(c) a plurality of substantially equi-spaced anchor members fixedlymounted on a platform beneath said body, said anchor members lying on acircle, or on an arc of a circle, the centre of said circle being saidvertical axis;

(d) means for engaging the end of said ram rod which is remote from saidram cylinder with each of said anchor means;

(e) substantially circular guiding means for guiding said end of saidram rod from one anchor means to another of said anchor means;

(f) locking means for temporarily locking said arm in locations occupiedby said arm in its rotational movement about said vertical axis; and

(g) hydraulic control means for expanding and contracting said hydraulicram.

For a better understanding of the present invention, embodiments thereofwill now be described, by way of example only, with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a perspective sketch of a dish antenna designed by the presentinventors for the collection of solar radiation.

FIG. 2 is a side elevational view of the antenna of FIG. 1.

FIG. 3 is a schematic exploded representation of part of the dishsupport frame of the antenna of FIGS. 1 and 2.

FIG. 4 is a sectional view at 4--4 of FIG. 3.

FIGS. 5, 6 and 7 illustrate three alternative adjacent tetrahedral strutassemblies that may be used in implementations of the present invention.

FIGS. 8, 9 and 10 are schematic drawings which illustrate the mechanismand method for rotating the antenna of FIGS. 1 and 2 about a verticalaxis.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The dish antenna 10 illustrated in FIGS. 1 and 2 was designed by thepresent inventors and has been assembled at The Australian NationalUniversity, in Canberra, Australia. It comprises a dish 12 constructedusing a number of reflecting dish segments or panels 31 which aremounted on a dish supporting frame 15. The dish 12 is a sphericalreflector having a hexagonal periphery. The shorter edges of thehexagonal periphery are 21.8 meters long and its longer edges are 24.7meters long. The dish has an aperture of 400 square meters. If this dishshould focus the sun (which subtends an angle of about 0.5° at theearth) sharply, the sun's image would have a diameter of approximately14 cm. Such an image of the sun would create such a concentration ofenergy at the focus that most materials used in equipment to harness thesun's energy would be damaged. Thus the dish 12 is designed to form a"fuzzy" image of the sun, having an area of about five times the sharplyfocused image, at its focal region. A "receiver" 13 of the solar energy,which is supported by struts 21, is positioned at the focal region ofthe dish 12. In the solar collector built by the present inventors, thereceiver 13 comprises a coiled tube which is used to generate highquality steam (that is, steam at high pressure and hightemperature--although the temperature of the steam is limited by itsapplication, for most steam turbines cannot accept steam at atemperature in excess of 500° C.).

It is emphasised that the embodiment of the present invention featuredin FIGS. 1 and 2 is but one example of an implementation of the presentinvention, and the present invention is not limited to solar energycollectors generally, or to antenna configurations which are similar tothat illustrated in FIGS. 1 and 2.

The dish supporting frame 15 is pivotally connected by a horizontal tiltaxis 14 to a base frame 11 of the antenna structure. The elevation ofthe line of sight 32 of the dish antenna is controlled by movement ofthe dish 12 and its support frame 15 about the tilt axis 14 using ahydraulic ram arrangement 16 which controls the movement of a sub-frame17 that extends from the dish support frame of the solar collectorantenna. However, this tilting arrangement could be substituted by anyother suitable drive mechanism, such as a screw drive, a rack and pinionmechanism, or a recirculating ball mechanism).

The base frame 11 of the antenna is mounted for rotation about avertical axis 20, which is at the centre of a circular track 18. Inconventional antennas, the rotation of the main or base frame 11 aboutthe axis 20 would be effected by drive motors, driving pinions whichengage a circular track. The solar collector antenna constructed by thepresent inventors utilises a different form of frame rotating mechanism,constructed in accordance with a second aspect of the present invention.

A part of the dish supporting frame 15 is illustrated in FIG. 3. FIG. 3shows a number of rigid struts, including struts 60, 61, 62, 63, 64, 65and 66, connected to nodes 50, 51, 52, 53 and 54 of the dish supportingframe. The nodes used by the present inventors are conventional nodes inthe form of spherical members on which planar surfaces are formed. Theplanar surfaces of each node are adapted to receive the ends of struts,which are rigidly attached to the node (for example, using a threadedextension to the strut which is screwed into a threaded bore in thenode).

The nodes 50, 51, 52, 53 and 54 of FIG. 3 also constitute mountingpoints for the dish segments 31. By carefully choosing the lengths ofthe struts of the dish supporting frame, the frame can be constructed sothat those nodes lie on the envelope of the required dish surface,which, in the case of the solar collection antenna of FIGS. 1 and 2, isa paraboloidal surface (which approximates closely to a sphericalsurface over much of the dish surface).

These nodes are interconnected to other nodes, not shown in FIG. 3. Adish support frame created by struts connected only to the nodes at theenvelope of the dish surface would form a weak structure. However,further nodes 55, 56 and 57 are provided away from the dish surface. Asshown in FIG. 4, node 55 is connected by struts to nodes 50, 51, 52, 56and another unshown node. Node 56 is connected by struts to nodes 51,52, 53, 55 and 57; node 57 is connected by struts to nodes 51, 53, 54,56 and another unshown node.

Thus nodes 50, 51, 52 and 55 and the struts connecting these nodes forma first tetrahedral strut assembly, and nodes 51, 52, 53 and 56 and thestruts connecting these nodes form a second tetrahedral strut assembly.Similarly, nodes 51, 53, 54 and 57 and the struts connecting these nodesdefine yet another tetrahedral strut assembly. Further tetrahedral strutassemblies are defined by each group of four adjacent nodes. It will beappreciated that these tetrahedral strut assemblies, by virtue of theirtriangulation in three dimensions and the interlinking of struts betweenadjacent tetrahedral structures provide an exceptionally strong andrigid structure which can be very light in weight when compared withconventional dish supporting frames and the heavy cross-braces used withsuch support frames in an attempt to provide rigidity.

It should be noted that the tetrahedral strut assemblies of the dishsupporting frame may be joined together with "face contact", "edgecontact", "point contact" or any combination of such contact types.

For example, as shown in FIG. 5, ABCD represents a first tetrahedralstrut assembly with great inherent rigidity. ACDE represents a secondtetrahedral strut assembly having its face ACD connected to face. ACD ofthe tetrahedral strut assembly ABCD. Note that all nodes A, C, D and Eof the tetrahedral strut assembly ACDE are absolutely fixed in relationto each other, and in relation to the tetrahedral ABCD. Thus these strutassemblies will fit together with precision and without requiringadjustment.

FIG. 6 shows two tetrahedral strut assemblies, ABCD and ACXY, joined inan "edge contact" configuration. It will be noted that there is nomember joining nodes D and X. Should such a connecting strut be present,there would be a third tetrahedral strut assembly ACDX in face contactwith tetrahedral strut assemblies ABCD and ACXY, on faces ACD and ACXrespectively.

Whilst the nodes of each tetrahedral strut assembly are accurately andrigidly located in each assembly, it will be noted that tetrahedralstrut assembly ACXY may move relative to strut assembly ABCD by rotationabout the line AC. By suitable design, adequate rigidity and precisionmay be provided which allows for the deletion of strut members, such asDX, with a further saving of cost and weight. It will also be noted thatduring the construction of the dish supporting frame, some additional(but temporary) support may be needed to locate some of the tetrahedralstrut assemblies prior to completion of the frame structure. (Thiscomment also applies to the beam and tower structure realisations of theinvention.)

FIG. 7 shows two tetrahedral strut assemblies, ABCD and CEFG, joinedtogether in point contact at node D. The two tetrahedral strutassemblies may rotate relative to each other about node C, but bysuitable design, adjacent tetrahedral strut assemblies will fix therelative positions of assemblies ABCD and CEFG, and will providesufficient rigidity whilst reducing the number of components and weightof the dish support frame. Again some additional, but temporary, supportwill be required during the assembly of the support frame.

The provision of a dish support frame for a dish antenna usingtetrahedral strut assemblies provides the following significantadvantages over the conventional frames currently used to supportdishes.

1. Stability of the reflecting surface. The space frame of the presentinvention is extremely rigid. Thus, irrespective of dish orientation orwind loadings, the reflecting surface remains substantially undistorted.

2. Rigidity and strength combined with low mass. The interlinking natureof the frame structure provides high rigidity and strength withoutexcessive mass, thus reducing the size and power requirements forsupport structures, foundations, and associated operating equipment.This, in turn, allows the economic construction of large aperturecollectors.

3. Easy manufacture and assembly. The dish supporting frame may bemanufactured utilising existing node system technologies. Once therequired dish has been designed, the appropriate nodes and struts may beproduced in a factory and transported to the site for in situ assemblyof the antenna. Due to the accuracy presently possible with such nodesystems, the dish support frame, and then the dish and the remainder ofthe antenna, may be easily assembled on site with low requirements forboth labour skills and time. Furthermore, once the support frame isassembled, the positions of the mounting points are sufficientlyaccurate for the reflecting panels to be secured directly to thesenodes, with no on-site adjustment of the dish surface being required.

4. Modularity. Once a dish antenna has been designed, the geometry ofthe nodes remains unaltered whatever size of dish constructed. Thusdifferent size dishes of the same design may be manufactured merely byaltering, by a given factor, the lengths of the struts connecting thenodes, or further "rings" of structural tetrahedral strut assemblies maybe added to the dish perimeter.

As noted above, the line of sight of the surface formed by thereflecting panels 31 is indicated by line 32. Conventional dish antennashave their dish support frames mounted for rotation about a horizontalaxis which intersects this major axis or line of sight 32, or liesadjacent to it.

The horizontal axis 14 about which the dish support frame of the presentinvention rotates is displaced a substantial distance from the majoraxis or line of sight 32. In the embodiment of FIGS. 1 and 2, the axis14 lies approximately midway between the centre of the surface 30 andits lowest outer edge 33. It will be appreciated that the stressesgenerated in the dish due to an offset axis of pivoting require asomewhat stronger dish support structure than would be the case with anaxis of rotation at approximately the balance point of the dish and itssupport frame. The use of a dish supporting frame comprising tetrahedralstrut assemblies provides such a frame, and allows it to be realisedeconomically.

Positioning the pivot axis 14 away from the centre of the collector dishmeans that, when aligned at or near the horizon, only about one quarterof the diameter of the collector lies below the axis 14. Thus a lowerbase frame 11 may be utilised, to provide a further saving in materialsand cost.

Offsetting the pivot axis 14 does not reduce the total height of theantenna when the collector dish is tracking near the horizon. Trackingnear the horizon, however, occurs early in the morning and late in theafternoon, when winds are usually light, and will have little effect ona large dish with a nearly horizontal line of sight. Stronger winds areusually experienced during the middle of the day, when maximum energy iscollected. At this time, the total height of an antenna with an offsetpivot axis 14 is reduced substantially and the collector dish remainsrelatively close to the ground. While this in itself does not effectcollector efficiency, the reduction in overall height does reduce thewind loads on the structure when the wind is stronger than a lightbreeze. This is partly due to the reduction in the area of the antennastructure that is exposed to the wind, and partly due to the normalattenuation of wind near to the ground. One consequence of this is thepossibility that the dish supporting frame may be made weaker and theantenna will still be operable in the same maximum wind speeds. However,it is preferable not to reduce the strength of the dish support frame,but have the benefit of the ability of the antenna to be operated inhigher wind speeds without the risk that the wind will generate loads onthe antenna structure which will damage the structure.

Thus, use of the off-set axis 14 feature of the invention firstly lowersthe forces applied to the antenna structure, which allows continuedoperation in higher wind speeds, and secondly lowers the overall heightwhen the collector is "parked", so that wind loads and potential damageare reduced. The only disadvantage of an off-set horizontal axis is aslightly higher drive energy requirement, which must now support part ofthe weight of the dish. However, the increased efficiency of operationof a solar collector antenna and the lower costs of construction morethan compensate for this.

The other supporting structures used in the antenna illustrated in FIGS.1 and 2, namely the base frame 11 and the back frame 25, are alsoconstructed using tetrahedral strut assemblies.

The dish support frame (and thus the dish) of the antenna shown in FIGS.1 and 2 may be rotated about the horizontal tilt axis 14 by any suitablemechanism, but preferably using the hydraulically controlled mechanism16, which is described in detail in the specification of Australianpatent application No PL 5900, filed 17 Nov. 1992. In addition, theentire antenna is rotated about the vertical axis 20, preferably usingthe "walking ram" mechanism which is described in more detail below,with reference to FIGS. 8, 9 and 10.

FIGS. 8, 9 and 10 are schematic views, from above, of the preferredarrangement for rotating the base frame 11 of the antenna of FIGS. 1 and2 about a vertical axis 82. The base frame is provided with an arm 81which extends generally radially from the axis 82. The arm 81 may be anintegral part of the frame. The cylinder 84 of a double acting hydraulicram is connected to the arm 81 at or near its free end. The hydraulicram is extended and the free end of the rod 86 of the ram is connectedto an anchor member 85a. Anchor member 85a is one of a plurality ofanchor members 85a, 85b, 85c, . . . which are rigidly mounted on acircle centred at the axis 82. The hydraulic ram is contracted, and theaction of moving the ram rod 86 into the ram cylinder 84 pulls the endof the arm 81 towards the anchor member 85a. Thus the frame which thearm 81 is attached (or of which the arm 81 forms a part) is rotated inthe direction of arrow 87 about the vertical axis 82.

At a predetermined contraction of the hydraulic ram, shown in FIG. 9,the contraction of the ram ceases. At this point, the arm 81 is lockedin its location and the end of the ram rod 86 which has been connectedto the anchor member 85a is released from that anchor member. In someinstances, it may not be necessary to lock or clamp the arm 81 in itslocation when the contraction of the ram ceases. The locking of the arm81 is necessary when a large dish antenna is being rotated because windforces on the antenna dish and its supporting structure may cause theantenna to rotate in the opposite direction to arrow 87.

The hydraulic ram is then expanded. This expansion causes the end of theram rod 86 to move along a generally circular guide 89 until it reachesa predetermined degree of expansion, shown in FIG. 10, which occurs whenthe free end of the ram rod reaches the next anchor member 85b. At thispoint, the ram rod is connected to the anchor member 85b, the arm 81 isunlocked, and the contraction of the hydraulic ram begins again, tocontinue the rotation of the arm 81 in the direction of arrow 87.

When the body to be rotated by this mechanism is a large dish antenna ofthe type illustrated on FIGS. 1 and 2, the "receiver" upon which thelight from the sun is focused is larger than the sun's "fuzzy" image.Typically the image of the sun remains within the receiver's target zonefor about 20 seconds, so that it is only necessary to adjust theposition of the collector every 20 seconds or so, when the image isabout to leave the receiver target zone.

Using the arrangement illustrated in FIGS. 8, 9 and 10, it has beenfound that it is convenient to contract the hydraulic ram in steps,every 15 seconds. This is easily achieved by controlling the volume offluid in each chamber of the hydraulic ram and does not require anyfeedback arrangement; the control of the position of the antenna'scollector is achieved by merely controlling the amount of fluid pumpedby a hydraulic motor. Furthermore, although the rotation of the arm 81in the direction of arrow 87 is a very slow rotation, when the end ofthe ram rod has to be moved from one anchor member to the next anchormember, the ram can be driven at a relatively high speed, such that thetime from reaching the end of travel and being disconnected from oneanchor member to the time of being connected to the adjacent anchormember and commencing the next contraction of the ram is only about 15seconds. Thus the time taken in moving the ram rod from one anchormember to the next does not result in the sun's image leaving thereceiver. A further advantage of this rapid movement is that, ifnecessary, the ram can off-steer the collector by a substantial amountby engaging an anchor then fully contracting in one operation instead ofin steps.

In practical implementations of the arrangement depicted in FIGS. 8, 9and 10 (for rotation of the base frame about a vertical axis), theanchor members are approximately equi-spaced and secured to a circularconcrete track. Each anchor member typically comprises a stanchion witha plate-like member at the top of the stanchion, extending radiallyinwards. A hole or aperture in the plate-like member is adapted toreceive a draw pin which is mounted for vertical movement at the end ofthe ram rod 86. When the draw pin and the aperture are aligned, movementof the draw pin into the aperture secures the end of the ram rod to theanchor member. The draw pin assembly is mounted on a roller structurewhich carries the end of the ram rod 86 when the ram is expanding andthe end of the ram rod is being moved from one anchor member to thenext. A guide mechanism is required to guide the roller structure andensure that the draw pin and the aperture in the plate-like memberbecome aligned.

The operation of the base frame rotating mechanisms which have beenbuilt by one of the present inventors, for the solar collector dishantenna featured in FIGS. 1 and 2, are controlled by a programmedmicroprocessor, which ensures that the line of sight of the collectordish of the antenna automatically tracks the sun each day.

Skilled engineers will appreciate that the embodiments of the presentinvention which are illustrated in the accompanying drawings anddescribed above are examples only of realisations of the presentinvention, and that the invention is not limited to those embodiments.

We claim:
 1. A dish support frame for the dish of a dish antenna, saiddish support frame comprising:(a) a plurality of strut assemblies, eachstrut assembly comprising six rigid struts, said struts having endsconnected to four nodes of the support frame to form a tetrahedral strutassembly; and (b) a plurality of dish mounting points, each dishmounting point being at a respective one of the nodes of a strutassembly, said dish mounting points being located on the envelope of theshape of the dish; said dish support frame further including a mechanismfor rotating said support frame about a vertical axis, said mechanismcomprising:(a) an arm attached to or forming part of said support frame,said arm extending generally radially from said vertical axis; (b) ahydraulic ram having a ram cylinder and a ram rod actuated by hydraulicfluid within said ram cylinder, said ram cylinder being connected tosaid arm; (c) a plurality of equi-spaced anchor members fixedly mountedon a platform beneath said support frame, said anchor members lying on acircle, or on an arc of a circle, said vertical axis passing through thecentre of said circle; (d) means for temporarily engaging the end ofsaid ram rod which is remote from said ram cylinder with a selected oneof said anchor members; (e) substantially circular guiding means forguiding said end of said ram rod from said selected anchor member to anadjacent anchor member; (f) locking means for temporarily locking saidarm in locations occupied by said arm in its rotational movement aboutsaid vertical axis; and (g) hydraulic control means for expanding andcontracting said hydraulic ram.
 2. A dish support frame as defined inclaim 1, in which the strut assemblies of at least one pair of adjacenttetrahedral strut assemblies have three struts and three nodes incommon, whereby said at least one pair of strut assemblies are joinedtogether by face contact of their adjacent tetrahedral structures.
 3. Adish support frame as defined in claim 1, in which the strut assembliesof at least one pair of adjacent strut assemblies have two nodes and onestrut in common, whereby said at least one pair of strut assemblies arejoined together by edge contact of their adjacent tetrahedralstructures.
 4. A dish support frame as defined in claim 1, in which thestrut assemblies of at least one pair of adjacent strut assemblies haveone node in common, whereby said at least one pair of strut assembliesare joined together by point contact of their adjacent tetrahedralstructures.
 5. A dish support frame as defined in claim 1, mounted on abase frame of the antenna for pivotal movement about a horizontal axis,said axis being displaced laterally from the centre of the dish supportframe.
 6. A dish support frame as defined in claim 5, in which said axisis located approximately mid-way between the centre of the dish supportframe and the edge region of the dish support frame.
 7. A dish supportframe as defined in claim 5, including a reflecting dish for directingsolar energy to a receiver which is mounted at the focal region of saidreflecting dish, said reflecting dish being mounted on said supportframe.
 8. A dish support frame as defined in claim 1, in which(1) saidplatform is the earth or concrete pad mounted on the earth; (2) each ofsaid anchor members is a stanchion affixed to said platform, saidstanchion having a plate member extending radially inwards with relationto said circle; (3) said end of said arm is supported by a carrieradapted to move over the surface of said platform; and (4) said meansfor temporarily engaging said end of said ram rod to an anchor membercomprises means for temporarily connecting said end of said arm to saidplate member.
 9. A dish support frame as defined in claim 1, including aprogrammed microprocessor for controlling the operation of saidmechanism.
 10. A dish support frame as defined in claim 1, including areflecting dish for directing solar energy to a receiver which ismounted at the focal region of said reflecting dish, said reflectingdish being mounted on said support frame.